This invention relates generally to improved rare-earth doped ceramic scintillator materials which are especially useful in medical radiographic applications as well as other x-ray, gamma radiation, and nuclear radiation detection applications. More specifically, the present invention relates to rare earth doped polycrystalline yttria-gadolinia (Y.sub.2 O.sub.3 -Gd.sub.2 O.sub.3) ceramic scintillators which have been treated during or after sintering to reduce radiation damage otherwise occurring when said scintillator material is exposed to the aforementioned type high energy radiation for the conversion of said radiation to a display image.
Solid state scintillator materials have long been used as radiation detectors to detect penetrating radiation in such applications as x-ray counters and image intensifiers. More recently, such detectors have played an important role in computerized tomography (CT) scanners, digital radiography (DR), and other x-ray, gamma radiation, ultraviolet radiation, and nuclear radiation detecting applications. The scintillator materials emit visible or near visible radiation when stimulated by x-rays or other high energy electromagnetic photons hence are widely incorporated as integral parts of various industrial and medical radiographic equipment. In medical applications, it is especially desirable that the scintillator output be as large as possible to minimize exposure of the medical patient to the x-ray dosage. A known class of scintillator materials considered for use in CT applications is monocrystalline inorganic compounds such as cesium iodide (CsI), bismuth germanate (Bi.sub.4 Ge.sub.3 O.sub.2), cadmium tunstate (CdWO.sub.4), calcium tunstate (CaWO.sub.4) and sodium iodide (NaI). Another known class of solid state scintillator materials comprises polycrystalline inorganic phosphors including europium activated barium fluorochloride (BaFCl:Eu), terbium activated lanthanum oxybromide (LaOBR:Tb) and thulium activated lanthanum oxybromide (LaOBr:Tm). A still third class of already known solid state scintillator materials found useful in computerized tomography comprises various dense sintered polycrystalline ceramics such as rare earth doped yttria-gadolinia (Y.sub.2 O.sub.3 /Gd.sub.2 O.sub.3) and polycrystalline forms of said previously mentioned phosphors including BaFCl:Eu, LaOBr:Tb, CsI:Tl, CaWO.sub.4, and CdWO.sub.4.
In the aforementioned commonly assigned copending application, there is disclosed a CT scanner employing a solid state scintillator body as the x-ray conversion means. Said prior art polycrystalline ceramic scintillator comprises between about 5 and 50 mole percent Gd.sub.2 O.sub.3, between about 0.02 and 12 mole percent of either Eu.sub.2 O.sub.3 or Nd.sub.2 O.sub.3 as a rare earth activator oxide, between about 0.003 and 0.5 mole percent of at least one afterglow reducer selected from the group consisting of Pr.sub.2 O.sub.3 and Tb.sub.2 O.sub.3, the remainder of said scintillator composition being Y.sub.2 O.sub.3. As recognized in connection with the CT scanner equipment further disclosed in said co-pending application, the scintillator material when excited by impinging X radiation emits optical wavelengths energy. In typical medical or industrial applications, the optical output from the scintillator material is made to impinge upon a photoelectrically responsive device which produces electrical output signals. The amplitude of these signals is directly related to the intensity of the impinging X radiation. The electrical signals can be digitzed for processing by digital computer means, which means generates the absorption co-efficients in a form suitable for a display on a cathode ray tube screen or other permanent media.
In order to meet the specific and demanding requirements of this type medical radiographic applications, the scintillator material employed must be an efficient converter of X radiation into optical radiation, in those regions of the electromagnetic spectrum which are most efficiently detected by photodetection means such as photomultipliers or photodiodes. It is also desirable that this scintillator materials transmit optical radiation efficiently, by avoiding optical trapping, so that optical radiation originating deep inside the scintillator body escapes for detection by the externally situated photodetectors. The scintillator materials should also have high x-ray stopping power, low hysteresis, spectral linearity, and short afterglow or persistance. High x-ray stopping power is desirable for efficient x-ray detection, because x-rays not absorbed by the scintillator escape detection. Hysteresis refers to the scintillator material property whereby the optical output varies for identical x-ray exitation, based on the irradiation history of the scintillator. For CT applications, typical detection accuracies are on the order of one part in one thousand, for a number of successive measurements taken at a relatively high rate. Accordingly, low hysteresis is required in order to provide repeated precise measurements of optical output from each scintillator body, and to provide substantially identical optical outputs for identical X radiation exposure impinging upon the scintillator body. Spectral linearity is important because x-rays impinging upon the scintillator body typically include a number of different frequencies, and because the scintillator response to the radiation should be substantially uniform for all such frequencies. Afterglow or persistance is the tendancy of the scintillator to continue emitting optical radiation for a time after termination of the x-ray exitation. Long afterglow results in blurring, with time, of the information bearing signal.
Polycrystalline ceramic scintillators exhibiting many of the desirable properties mentioned above are further described in U.S. Pat. No. 4,421,671, also assigned to the present assignee. The scintillator described therein are comprised of yttria and gadolinia, and include at least one of a variety of rare earth activators for enhancing luminescent efficiency. The scintillator composition may also include one or more of several disclosed transparancy promoter and light output restorers. Said aforementioned patent still further discloses that luminescent afterglow of the yttria-gadolinia ceramic scintillators described therein may be reduced by adding ytterbium oxide (Yb.sub.2 O.sub.3), strontium oxide (SrO), or calcium oxide (CaO).
Radiographic equipment employing said prior art solid state scintillator materials for the conversion of high energy radiation to an optical image still experience efficiency loss after exposure to high dosages of radiation. For example, radiation damage resulting for bismuth germanate single crystal scintillators reports an 11% damage occurring after a thirty minute exposure to ultraviolet radiation from a mercury lamp. A similar result occurs for higher energy gamma radiation. Furthermore, the variation in radiation damage from crystal to crystal of bismuth germanate is reported to be quite high, approximately a factor of 30. A similar efficiency loss is found when the above mentioned polycrystalline type ceramic scintillators are exposed to high energy radiation dosages. The radiation damage in said type scintillator materials is characterized by loss in light output and/or a darkening in color with prolonged exposure to radiation, and this decreased light output is found to be variable in magnitude from batch-to-batch. For example, yttria-gadolinia ceramic scintillators activated with europium exhibit a reduction in light output of 4 to 33%, depending upon the batch, for 450 roentgens of 140 kVP x-rays. This amount and variation of x-ray damage is undesirable in a quantitative x-ray detector and must be minimized in order to avoid ghost images from prior scans.
It remains desirable, therefore, to provide a polycrystalline solid state scintillator material which is not subject to damage produced from exposure to high radiation dosages.
It is another important object of the present invention to modify already known efficient polycrystalline solid state scintillator materials so as to better resist changes in scintillator efficiency with increasing x-ray dosage.
It is still another important object of the present invention to provide a method for preparing such improved polycrystalline type solid state scintillator materials.
Still another important object of the present invention is to provide improved radiographic equipment and methods utilizing said presently modified polycrystalline type solid state scintillator materials.